Manholes in a roadway that are connected to a sewage disposal system are enclosed with a cast iron lid or cover. In the past, the manhole lids have been provided with one or more vent holes for the purpose of venting sewer gases through the lid to the atmosphere. However, it has been found that during heavy rains or flooding conditions, surface water will enter the manholes through the vent holes in the lid, as well as through the joint between the lid and the manhole frame, and the added surface water can seriously overburden the sewage system. Under these conditions, the excess effluent containing raw sewage is discharged directly into lakes or streams without treatment.
Because of this, the recent trend has been to seal the vent holes in the lid, as well as sealing the lid to the manhole frame, to prevent surface water from entering the sewage system. The lid rests on a shoulder or ledge formed in the cast iron manhole frame, and in one proposed sealing system, an O-ring seal is inserted within a groove formed in the under surface of the lid. The O-ring bears against the ledge on the frame to provide a seal between the lid and the frame.
In service, there is considerable rotation of the manhole lid relative to the frame due to vehicular traffic on the roadway. Vehicle tires contacting one edge of the lid and accelerating or deaccelerating will cause the lid to rotate. With the use of an O-ring seal, as previously described, the seal can be worn or torn due to the rotation.
A further problem with the O-ring seal arises during removal and replacement of the lid from the frame. The lid is normally removed by inserting a pick or other instrument into a recess in the lid to pry the edge of the lid upwardly and the lid is then pulled across the frame for complete removal. Dragging the lid across the frame can cause the O-ring seal to rip or tear, and in some cases the seal can be completely dislodged from the groove and may be lost down the sewer. A seal accidentally dropped into the sewer can clog the sewer or damage pumps.